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Cetacean passive acoustic network

A passive acoustic hydrophone network has been put into operation to record the presence of porpoises and dolphins in the Belgian part of the North Sea (BPNS).

Scientific background

Porpoises and dolphins (Odontoceti) use echolocation to extract information from their surroundings. Dolphins produce clicks in a wide frequency range and are typically short and loud while porpoises are producing longer and weaker clicks in a narrow frequency range (120 - 145 kHz, mode 132 kHz). These clicks can be recorded by the passive acoustic device, C-POD (Chelonia Limited, figure below left), when a marine mammal is swimming in the vicinity of the recorder. The C-POD can record clicks between 20 and 160 kHz including ambient background noise, sonar and other biotic underwater sound. The key to the performance of the C-POD is detection and classification of series of clicks, so-called click trains. Click trains have distinctive features which are used by the classification algorithms to identify the occurring cetacean species.

Infrastructure

The development of a permanent network of C-PODs in the BPNS, as part of the marine LifeWatch observatory, started in 2014. Ten C-PODs are now continuously deployed on tripod frames positioned along an east-west gradient covering coastal, midshore and offshore zones of the BPNS. The C-POD itself is attached to a buoy with an an acoustic release system (Vemco VR2AR), allowing for the recovery of the equipment. Before 2018, C-PODs were connected to navigation buoys or anchored to ship wrecks or to artificial hard structures .